In recent years, as aging has progressed, problems of infectious disease in elderly people with deterioration in bodily functions such as immunocompetence have been attracting attention. As one example, fungal nail disease, for which a superficial fungus is the causative microorganism, can be cited. This is a fungal disease that is caused by propagation in the nail of a fungus such as Trichophyton mentagrophytes that normally causes a fungal disease in the skin. In such a fungal nail disease, since the nail is a hard tissue formed from keratin, it is difficult for an antifungal agent or a composition containing an antifungal agent used for a cutaneous fungal disease to be absorbed in the nail, and there are many problems in treatment involving administering a drug via the nail. Because of this, in the current situation it is difficult to develop a drug useful for onychomycosis, and as one contributory factor the absence of a suitable animal model for onychomycosis can be cited.
For example, as an infected animal model for evaluating the effect of an antifungal agent toward onychomycosis, a method in which a nail of a guinea pig is damaged by abrading with sandpaper, etc. and this is closed-patched with a dispersion of fungal arthroconidia (Patent Document 1), etc, is known, but in such a method the fungus is attached only to the surface of the nail, and although the fungus is present on the nail, it is difficult to say that onychomycosis, which is accompanied by degeneration in the interior of the nail and inhibits distribution of a drug, is reproduced. In such a model, since the fungus is present only on the nail surface, no cloudy part is formed in the nail, the area within the nail in which the fungus grows and is distributed is very small, and it is not an onychomycosis model in which the fungus has infiltrated into a deep part of the nail.
Other than the above, there are several reports relating to methods for infecting an animal with a fungus (Non-Patent Documents 1 to 3), but the period taken for infection is 60 days or longer, which is very long; moreover, the infection intensity is low and the fungus does not infiltrate sufficiently into a deep part of the nail, and this is not satisfactory in terms of producing an infection model that reflects the clinical manifestation with good reproducibility. It is thought that this is due to the structure of the nail and selection characteristics of the Trichophyton host and infestation site, but such onychomycosis and infected animal models cannot sufficiently function as evaluation systems for an antifungal agent. Furthermore, there are several problems with fungal nail disease as described above, and in spite of there being problems with the evaluation period and the reproducibility, there has been hardly any investigation into an infected animal model in order to solve the problems.
On the other hand, as a method for preparing an infected animal model, a method for infecting an animal with a fungus after compromising the immunity of the animal by administration of a steroid so as to put it in an easily infected state has been reported (Non-Patent Document 4), but it is clear that infection is affected by compromising the immunity, it is limited to an infected animal model in which infection is caused in the eyeball or mucosal tissue around the eye, where establishment of infection is very easy, and there are no examples of it being applied to the nail.
As hereinbefore described, the current situation is that neither an evaluation model appropriate for onychomycosis, etc. nor an accurate evaluation method therefor has yet been established.    Patent Document 1: International Patent Application WO 2001/7643    Patent Document 2: JP, A, 2001-128696    Patent Document 3: JP, A, 2002-65695    Patent Document 4: JP, A, 2001-133449    Non-Patent Document 1: Antimicrobial agents and chemotherapy, 46 (12), p. 3797-3801 (2002)    Non-Patent Document 2: Microbiol. Immunol., 47 (2), p 143-146 (2003)    Non-Patent Document 3: Mycoses, 48, p 108-113 (2004)    Non-Patent Document 4: Jpn. J. Infect Dis., 60, p. 33-39 (2007)